Where does the unused heat from a thermionic power converter go?

In a thermionic power converter, thermal energy is supplied to a hot electrode (the emitter) to liberate electrons, which then flow to a cooler electrode (the collector). While a portion of this thermal energy is converted directly into electrical energy, the remainder heats the collector.1

To maintain the collector at a suitably low temperature—which is necessary for the device to function—this excess heat must be removed. This process is essential because the converter operates as a heat engine, and the efficiency of the system depends on maintaining a temperature differential between the hot emitter and the cooler collector. In practical applications, such as those used in spacecraft, this waste heat is typically dissipated into the surrounding environment or managed through thermal management systems to ensure the collector remains at the required lower temperature.1